Transformation may take place in several stages in sequence, starting at a power station where voltage is increased to extra high voltage, such as 765 kV, 345 kV or 154 kV, for transmission purposes and is then progressively reduced to the voltage required for household or industrial use. That is, at a transformer substation, the extra high voltage is transformed to a line voltage of 22,900V, which corresponds to a phase voltage of 13,200V, for use in buildings or plants. The line voltage at the transformer substation may be further reduced to a voltage of 220/380V for household or industrial use by a transformer installed at a pole or a ground.
A transformer for the above-mentioned power supply is generally a one-end grounded auto-transformer which is connected to each of R-, S- and T-phases. The transformer has three taps of 13,800V, 13,200V and 12,600V at its primary winding. At this time, the secondary winding voltage is reduced to 230V and is supplied to a load. The secondary winding voltage is determined by changing the taps of the primary winding.
Power supply from a transformer substation to loads, such as houses or plants, using the transformer is described with reference to FIG. 1a. FIG. 1a illustrates the operation of transformer in which voltage drop segments are shown on a distribution line.
Referring to FIG. 1a, a distribution line 1 is typically 5 to 30 km in length from a transformer substation to a terminal 3 of the distribution line 1. An involved transformer substation 4 is connected to the terminal 3 in preparation for line fault.
Additionally, a connection switch 5 is provided on the distribution line 1 to distribute power, which is switched off at ordinary times and is switched on at a line fault. When the connection switch 5 is switched off, a load rate varies throughout the distribution line 1, resulting in different system voltages. Accordingly, a transformer 7 is set to have a primary winding voltage of 13,200V in a segment 6 with a voltage drop of 0 to 5%, and a transformer 9 is set to have a primary winding voltage of 12,600V in a segment 8 with a voltage drop of 5 to 10%.
However, the distribution line 1, of which voltage is controlled by an Under Load Tap Changer (ULTC) of a main transformer installed at the transformer substation 2, has a varying voltage of 23,816 to 22,670V (a ULTC voltage reference of 22.9 kV with a margin of +4 to −1%), which is drawn out of the transformer substation 2, depending on power service areas and loads at a peak time and at midnight. Therefore, since the voltage of the distribution line 1 having different characteristics in loads needs to be simultaneously adjusted, consumers neighboring the terminal 3 of the distribution line 1 may not be supplied with appropriate voltage.
Additionally, unlike the ULTC of the main transformer installed at the transformer substation 2, the transformers 7 and 9, which are installed at the distribution line 1 and convert the voltage to commercial voltage to distribute power to the consumers, manually change the voltage.
FIG. 1b is a partial cross-sectional view of such a conventional transformer which manually changes the voltage. As shown in FIG. 1b, the transformer includes a transformer cell 10 consisting of primary and secondary windings wound around an iron core, in which the primary winding is electrically connected to a tap switch 11.
FIG. 1c illustrates the transformer cell 10 of which primary and secondary windings 10-1 and 10-2 are electrically connected to the tap switch 11.
Referring to FIG. 1c, the tap switch 11 includes of first, second and third taps 11-1, 11-2 and 11-3, which can be connected to the primary winding 10-1 to set 12,600V, 13,200V and 13,800V, respectively. In this case, the voltage of 13,200V is set by manually switching off the first and third taps 11-1 and 11-3 and manually switching on the second tap 11-2 on the distribution line with a voltage drop between 0 and 5%. The voltage of 13,800V is set by manually switching off the first and second taps 11-1 and 11-2 and manually switching on the third tap 11-3 on the distribution line with a voltage drop between 5 and 10%. Likewise, the voltage of 12,600V is set by manually switching off the second and third taps 11-2 and 11-3 and manually switching on the first tap 11-1. Accordingly, the voltage can be adjusted at the secondary winding 10-2 of the transformer by switching on the respective taps 11-1, 11-2 and 11-3.
However, switching the taps may cause power-supply interruption between the transformer and the consumers since only one of the first, second and third taps 11-1, 11-2 and 11-3 is designed to be selected to prevent the transformer from being short-circuited when the first and second taps 11-1 and 11-2 are simultaneously switched on in the primary winding 10-1 having a voltage difference of 600V between the points of 13,800V, 13,200V and 12,600V.
Furthermore, the tap switching involves manual operation of the transformer while a cutout switch (COS) of the primary winding 10-1 is switched off. At this time, foreign matter, such as moisture, dust and rainwater, may come inside the transformer and deteriorate insulation oil contained in the transformer, causing trouble with the transformer.
Additionally, electric current which is reversely supplied from the generator to the secondary winding 10-2 during the tap switching may give operators or workers an electric shock. To prevent the electric shock due to the electric current, the secondary winding 10-2 needs to be grounded and lead wires connected to the secondary winding 10-2 need to be removed, which causes the operators to avoid the tap switching.
Furthermore, dispersed power sources, such as solar power, wind power, and cogeneration power, are increasingly involved with the distribution lines for power supply, leading to a severe fluctuation in voltage. In addition, it is difficult to adjust the tap switches installed at the transformer during power suspension in each voltage drop segment of the distribution line to prepare for seasonal loads or midnight loads of which characteristics are hard to estimate.